The modified thermoplastic polymer compositions of the present invention are particularly well suited to blow-molding processes, by which thin-walled articles such as bottles of all sizes are formed from a partially shaped, usually hollow polymer article known as a parison. The parison is formed by well-known processes such as extrusion or injection molding; it is then typically placed in a final mold, expanded by gas pressure to conform to the shape of the final mold and cooled to fix its shape. Variations of this process are well known in the art, and it may be used with many thermoplastic polymers. Such polymers that have been used by others to form blow-molded articles include poly(vinyl chloride), or PVC, poly(ethylene terephthalate), or PET, and polypropylene.
Desirably, such polymers balance melt-rheology properties such as flow and sag: the polymer must flow readily enough to be extruded, injection molded or otherwise formed into the parison; it must be sufficiently elastic and thermoplastic to fill the final mold readily under air pressure and heat, and without melt fracture or other surface distortion; yet it must be sufficiently resistant to flow or sag while cooling that the shape of the finished article is retained.
Further, if the polymer may be crystallized, the various processing, blending, and forming operations to which it is subjected must not accelerate crystallization to the point that blow-molding properties are degraded.
This combination of properties is difficult to find in unmodified polymers. Poly(vinyl chloride) may be easily modified with polymers that act as processing aids to make a polymer that is tractable in blow-molding applications, but other polymers have been more difficult to modify satisfactorily. Condensation polymers such as polycarbonates and polyamides and relatively low-molecular weight polymers such as polyethylene terephthalate of molecular weights in the range below about 20,000 have been difficult to modify for blow molding, and polycarbonate resins have proved especially difficult. It further has been difficult to blow-mold blends of engineering resins, such as polycarbonates with aromatic polyesters or with nylon, where both components are of relatively low molecular weight and low melt viscosity in their molten forms.
One approach that has been used to improve the blow-molding properties of polycarbonate resins has been to introduce chain branching into the polycarbonate molecule. Another has been to copolymerize the polycarbonate with a polyester. Neither of these approaches has been entirely successful; particular properties are improved, but the balance of properties important to blow molding is not sufficiently improved.
Branching or increasing the molecular weight of the polymer have been applied to other polymers used in blow molding. Branching is taught for poly(ethylene terephthalate), but requires careful control of melt reactivity to avoid causing processing times to be extended. Polyamides having reactive amine end groups may be reacted with groups on an additive, to tie together the polyamide molecules and effectively raise the molecular weight. This method requires careful control of stoichiometry, and may not be suited to less reactive polymers.
The rheology of polycarbonates has been controlled by additives, but the effects found do not corrolate with the improvement in low-shear and high-shear viscosity taught in the present invention.
Styrene-containing copolymers have been added to polycarbonate resins or polyester-polycarbonate blends as impact modifiers; these copolymers typically possess a core-shell (multi-stage) morphology, and the soluble portions of these copolymers have relatively low molecular weights, generally below about 300,000. Such impact-modifying polymers preferably contain a core (first stage) of rubbery poly(alkyl acrylate) or poly(butadiene) polymer or copolymer which is optionally crosslinked and/or graftlinked, and a thermoplastic hard shell (outer stage) of poly(styrene-co-acrylonitrile) copolymer.
Other impact modifying polymers are methacrylate-butadiene-styrene resins, which are multi-stage polymers having a butadiene polymer or copolymer, optionally containing vinylaromatics, as for examples styrenics, (meth)acrylate esters, or (meth)acrylonitrile, at levels below 30% and optional crosslinking, as a first stage. One or more thermoplastic methyl methacrylate polymer stages containing styrene, lower alkyl (meth)acrylates and/or (meth)acrylonitrile and optionally other monovinyl, monovinylidene, polyvinyl and/or poly vinylidene components are polymerized onto the first stage. Such modifiers are useful for impact-property modification of polycarbonates and polyesters.
Similarly, staged copolymers of crosslinked poly(alkyl acrylates) core//poly(alkyl methacrylates) shell have been combined with polycarbonates, polyesters, polyamides, and other engineering resins. Such core/shell polymers do not contain the high-molecular-weight vinylaromatic polymer of the present invention; the molecular weight of the extractable poly(alkyl methacrylate) phase is less than 500,000, and the remainder is crosslinked polymer. Such polymers do not function as melt rheology modifiers.
High-molecular-weight polymers have been added to various polymers, as for instance the addition of high-molecular-weight styrene to thermoplastic polystyrene as a foaming-process aid, or the use of high-molecular-weight copolymers of styrene with a minor amount of a nitrile or (meth)acrylic ester, in combination with low-molecular-weight copolymers of styrene with nitrile or (meth)acrylic ester and graft polymers of styrene-methyl methacrylate on a rubbery polymer, for the purpose of raising the softening temperature of polycarbonate resins.
It has not been disclosed that any of such high-molecular-weight polymers will affect the melt rheology of other engineering resins or blends in a way which makes feasible blow molding and other fabrication technology requiring good melt strength at low shear rates.
An object of the present invention is to provide a process for improving the rheological properties of thermoplastic polymer melts, and particularly the blow-molding properties of such melts. A further object is to provide a polymeric additive which improves these rheological properties. Additional objects will be apparent from the disclosure below.